The present invention relates to the field of light emitting diode based lighting and more particularly to a system for powering and controlling a plurality of LED strings having a controllable power source.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including, but not limited to, backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a monitor. In a large LCD monitor the LEDs are typically supplied in one or more strings of serially connected LEDs, thus sharing a common current.
In order supply a white backlight for the monitor, one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen as a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the monitor, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused so as to avoid hot spots by a diffuser. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need for a controlled white point.
Each of the colored LED strings is typically controlled by both amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance and color balance. AM is typically used to set the white point produced by the disparate colored LED strings by setting the constant current flow through the LED strings to a value determined as part of a white point calibration process and PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed on, is held constant to maintain the white point produced by the combination of disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the backlight by adjusting the average current over time. The PWM duty cycle of each color is further modified to maintain the white point, preferably responsive to a color sensor. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the white point. There is however a limit to the range of the PWM duty cycle and unfortunately when it has been reached, the maximum luminance begins to decline.
Each of the disparate colored LED strings has a voltage requirement associated with the forward voltage drop of the LEDs and the number of LEDs in the LED string. In the event that multiple LED strings of each color are used, the voltage drop across strings of the same color having the same number of LEDs per string may also vary due to manufacturing tolerances and temperature differences. Ideally, separate power sources are supplied for each LED string, the power sources being adapted to adjust their voltage output to be in line with voltage drop across the associated LED string. Such a large plurality of power sources effectively minimizes excess power dissipation however the requirement for a large plurality of power sources is costly.
An alternative solution, which reduces the number of power sources required, is to supply a single power source for each color. Thus a plurality of LED strings of a single color is driven by a single power source, and the number of power sources required is reduced to the number of different colors, i.e. typically to 3. Unfortunately, since as indicated above different LED strings of the same color may exhibit different voltage drops, such a solution further requires an active element in series with each LED string to compensate for the different voltage drops so as to ensure an essentially equal current through each of the LED strings of the same color.
In one embodiment, in which a single power source is used for a plurality of LED strings of a single color, power through each of the LED strings is controlled by a single controller chip, the controller chip exhibiting a dissipative active element operative to compensate for the different voltage drops. Unfortunately, the dissipative elements limit the range of operation of the controller chip, since the dissipative elements are a significant source of heat. Placing the dissipative elements external of the controller chip solves the problem of heat but unfortunately results in a higher cost and footprint and is thus less than optimal. In summary, a controller chip comprising within dissipative elements is limited by thermal constraints at least partially as a result of the action of the dissipative elements, yet still must provide both AM and PWM modulation.
As the LED strings age, their voltage drops change. Furthermore, the voltage drops of the LED strings are a function of temperature, and thus the voltage output of the power source must initially be set high enough so as to supply sufficient voltage over the operational life of the LED strings taking into account a range of operating temperatures. Utilizing a single fixed voltage power source for each color thus results in excess power dissipation, as the power source is set to supply a sufficient voltage for all the LED strings over their operational life, which must be dissipated for LED strings exhibiting a lower voltage drop.
What is needed, and not provided by the prior art, is a means for controlling the current flow through a plurality of LED strings responsive to thermal constraints.